Hi Emily!

Here's an image I've made that will hopefully make this question easier for you:

https://i.ibb.co/250PCFC/Capture.png. The key with this question is that everything has to be calculated RELATIVE TO THE FULCRUM. The fulcrum is where we are hanging the meter stick, which is at the 30 cm mark. So, I will calculate all of my distances RELATIVE to this value.

For example, even though the center of mass is at the 50-cm mark, it is 20 cm from the fulcrum!

Setting up a torque balance around that point, we get

torque on the left = torque on the right

(F)(20) = (5)(20) + (2)(50)

20F = 100 + 100

F = 200/20 = 10

Solving for the mass, we get

F = mg

m = F/g = 10/10 = 1 kg

The correct answer is D.

Let me know if that helps! ðŸ˜„